1. Field of the Invention
This invention relates to a control apparatus of an automatic transmission and more particularly to a control apparatus of an automatic transmission having an overdrive (hereinafter referred to as “MOD”).
2. Description of the Related Art
An automatic transmission in related art generally has a transmission having a transmission gear ratio of two to four stages and the fourth stage is put into OD designed so that the number of revolutions of output shaft becomes greater than the engine speed. Control is performed so that the OD gear of the fourth speed is entered under a predetermined condition of given speed or more during running in the drive range and fuel economy performance and quiet can be enhanced. A vehicle is provided with an OD off switch to inhibit gear change to the OD to intentionally exert the engine brake or perform smooth driving with a small number of gear change times in a control apparatus of an automatic transmission having such an OD.
FIG. 13 is a schematic drawing to show the operation state of the OD off switch placed in the vehicle. The OD off switch is placed on a shift lever, etc., and most OD off switches are of lock-type pushbutton switch form. As shown in FIG. 13(a), when the OD off switch usually is pushed and locked, the OD on state in which gear change to the OD is enabled is entered. As shown in FIG. 13(b), if the switch in the state in FIG. 13(a) is again pushed and unlocked, the OD off state in which gear change to the OD is inhibited is entered, and control for inhibiting gear change to the OD is started. At the same time, an ON/OFF indicator lamp in an instrument panel is lit. To restore to the OD on state from the OD off state, the driver again pushes the OD off switch to restore to the state in FIG. 13(a), whereby the OD off state is released and gear change to the OD is enabled and at the same time, the ON/OFF indicator lamp is switched off.
The OD off switch is provided, whereby the driver can set the switch intentionally so as to inhibit gear change to the OD; to provide the driving force with a margin by transmission control to the third speed at the acceleration time of passing, etc., or exert the engine brake at the deceleration time on a flat road or on a hill, the driver can intentionally inhibit gear change to the OD.
However, in the method in the related art wherein switch control between enabling and inhibition of gear change to the OD is performed by operating the OD off switch, once the OD off switch is operated for setting the OD off state, the state in which gear change to the OD is inhibited continues until the OD off switch is again operated for releasing inhibition of gear change to the OD and restoring to the OD on state. Thus, if the driver forgets about restoring to the OD on state after operating the OD off switch for setting the OD off state, the state in which gear change to the OD is inhibited continues and transmission control is performed at the transmission stage up to the third speed. Thus, the fuel economy performance is degraded, the content of hazardous substance in exhaust gas is also increased, and the environment is also adversely affected; this is a problem.
In the USA, it is planned that further enhancement of emission control and fuel economy improvement control will be conducted in the near term. To deal with the legal controls, if the driver forgets about restoring to the OD on state from the OD off state and drives the vehicle with the OD off state, when the driver once stops the engine and then restarts the engine, an apparatus for controlling so as to automatically release the OD off state and restore to the usual OD on state is also developed. According to such an apparatus for performing control so as to release the OD off state and restore to the OD on state at the restarting time after the engine stops, if the driver forgets about restoring to the OD on state by operating the OD off switch, the OD off state can be automatically released at the restarting time after the engine stops. However, the OD off state continues until the driver who forgets about restoring to the OD on state stops the engine after setting the OD off state by operating the OD off switch. Environment improvement made by improving the fuel economy performance for energy saving and reducing the hazardous substance in exhaust gas cannot sufficiently be intended; this is a problem.